try 50Hz rep rate of 1-2 mS positive pulse 0-5v
20 ms period = 50 Hz PWM frequency but this can vary and is NOT critical
kHz may be way too high might produce weird result such as you have
traditional PWM R/C signal is 1ms to 2ms with 1.5ms as neutral.
Servos swing 90 degrees on either side of neutral
-90 deg = 1 mS
neutral = 1.5mS (define a +-few% say 20 - 40 us deadband to avoid chatter)
+90deg = 2.0 mS
For a 555 based hardware driver solution:
http://pcbheaven.com/wikipages/How_RC_Servos_Works/